1. Field of the Invention
The present invention relates generally to the art of combustion of waste products, and particularly to the combustion of waste sludge products which have a high ash content, large particle content and high percentage content of water. Even more particularly, the present invention relates to the use of combustion as a viable use of dissolved air flotation sludge from animal rendering and food processing as a fuel.
2. Description of Related Art
A common method of waste water treatment in poultry processing plants, and other processing plants in which animal rendering is performed, is the dissolved air flotation system (DAF). A DAF system separates impurities from the waste water through an agitation and skimming process. A medium sized poultry processing plant, for example, can use up to one million gallons of water per day that must be processed with the DAF system. The contaminants from the water are expelled from the treatment plant as DAF waste sludge. It is not unusual for a one million gallon per day DAF system to generate 10,000 gallons of waste sludge daily.
In the past, this sludge has been spread or applied to land and allowed to dry, thereby becoming a fertilizer, if properly applied. In addition, deep well injection has been used. Problems have arisen however, because of the hazardous waste questions that are involved with placing the sludge into or on the ground. In particular, nitrogen and ammonia build ups when improper application is performed have ruined some tracts of land. Due to tightening governmental restrictions on land applications, deep well injection and other methods previously used to dispose of the waste sludge, animal rendering processing industries are in need of new technologies to address the DAF sludge disposal problem. Experts in the poultry processing industry regard the disposal of DAF waste sludge as one of the major problems facing the industry today and in the future.
The properties of DAF sludge are dependent upon the efficiency of the DAF unit and the type of rendering operation used by the processing plant. Plants can either produce high quality pet food additives from the rendering process, or lower quality feed additives. If poultry plants have on-site rendering, it is desirable to produce the higher quality pet food additives that bring a higher price per pound from their rendering process. By producing pet food quality additives, however, the plant must place tighter restrictions on the materials that can be handled in the rendering process. This in turn increases the quantity of waste in the water stream that must be handled by waste water treatment.
The waste sludge produced by such an operation tends to be much lower in water content than sludge from a plant that is not rendering for pet food additives. This is a result of the increased amount of fat and protein typically found in the waste water that are absorbed in the pet food additive. The sludge typically contains from seventy percent (70%) to ninety percent (90%) water, depending on the efficiency of the DAF system. The remaining portion of the sludge is primarily animal fat with high amounts of nitrogen and solids in the form of ash. In the preferred embodiment of the present invention, the system is capable of processing sludge having a water content up to ninety-nine percent (99%).
At eighty-seven percent (87%) water content, the heating value of the fat portion of the sludge contributes primarily to converting the water portion of the sludge to steam during combustion. In other words, 87% water content is the point where available heat input of the fat in the sludge does not add any net energy to the system. This does not necessarily exclude high water content sludges from combustion systems. In those situations in which high water content sludges are treated, the treatment in one of disposal rather than heat recovery from a fuel.
The inventors are unaware of any incinerator, boiler or other system capable of efficiently treating DAF sludge in an automatic, modulating system on an industrial level. To their knowledge, the only viable methods of large-scale disposal of the DAF sludge have been by land application, deep well injection and in some instances drying and injection into a solid waste incinerator. A system which could efficiently and effectively treat the DAF sludge and provide, in many circumstances, energy to the processing plant would be a significant improvement in the treatment of waste sludges, especially in animal rendering operations.